3.
POSITRON SOURCE
3.1
Overview
The ILC Positron Source generates the positron beam. The production scheme uses the
electron main linac beam passing through a long helical undulator to generate a multiMeV photon drive beam which then impinges onto a thin metal target to generate
positrons in electromagnetic showers. The positrons are captured, accelerated, separated
from the shower constituents and un-used drive beam photons and transported to the
Damping Rings. The baseline design is for 30% polarized positrons. There are spin
rotators before injection into the damping rings to preserve the polarization and there is
also sufficient beamline space to allow for an upgrade to a polarization of
~60%.(citations)
The positron source performs several critical functions:
• generation of a high power multi-MeV photon production beam. This requires
suitable short period, high K-value helical undulators.
• production of the positron bunches in a metal target that can reliably deal with
the beam power and radioactive environment induced by the production
process. This requires high power target systems.
• capture, accelerate and transport of the positron bunch to the Damping Rings
with minimal beam loss. This requires high gradient normal conducting RF and
special magnets to efficiently capture the positrons. The long transport lines also
require large aperture magnets to efficiently transport the large transverse
emittance positron beams.
The Positron Source also has sufficient instrumentation, diagnostics and feedback
(feedforward) systems to ensure optimal operation.
3.2
Beam Parameters
The key parameters of the Positron Source are given in Table 3.1. The source produces 2
x 1010 positrons per bunch at the IP with the nominal ILC bunch structure and pulse
repetition rate. It is designed with a 50% overhead and can deliver up to 3 x 1010 at
injection into the 0.075 m-rad transverse damping ring dynamic aperture. The main
electron linac beam has energy of 150-250 GeV and passes through ~150 meters long
helical undulator, with a period of 1.15 cm and a K value of 0.92. For the 150GeV
nominal drive beam, the first harmonic cut-off of the photon drive beam is 10.1 MeV and
the beam power is ~63 kW. Approximately 4.4 kW of this power is deposited in the
target in an area ~ 1 mm rms. A windowless high speed rotaing target is required to
handle the high beam power and heat deposition.
The Positron Source undulator is long enough to provide adequate yield for electron
beam energy over 150 GeV. For lower energy operation, the electron complex operates at
a 10 Hz repetition rate with 5 Hz of 150 GeV electrons on the target to produce positrons
and 5 Hz of electrons at the desired energy for collisions.
Table 3.1
Nominal Positron Source Parameters
Parameter
Positrons per bunch at IP
Bunches per pulse
Pulse Repetition Rate
Positron Energy (DR injection)
DR Dynamic Aperture
DR Energy Acceptance
DR Longitudinal Acceptance
Electron Drive Beam Energy(*)
Undulator Period
Undulator Strength(**)
Symbol
Value
Units
nb
Nb
frep
E0
Δ
Al
Ee

2 x 1010
1312
5
5
<0.07
0.75
3.4 x 37.5
150/175/250
1.15
number
number
Hz
GeV
m-rad
%
cm-MeV
GeV
cm
K
0.92/0.75/0.45
-
γ(Ax +Ay)
Undulator Type
Helical
Undulator Length
Lu
147
m
Photon Energy (1st harm
Ec10
10.1/16.2/42.8
MeV
cutoff)
Photon Beam Power
63.1/54.7/41.7
kW
P
Target Material
Ti-6%Al-4%V
Target Thickness
Lt
0.4 / 1.4
r.l. / cm
Target Absorption
7/7.2/5
%
Incident Spot Size on Target
1.4/1.2/0.8
mm, rms
i
Positron Polarization
P
31, 30,29
%
*: For CM=200GeV, 230GeV and 250GeV, the machine operates in 10Hz mode where a
5Hz 150GeV beam is dedicated for the drive beam of positron source.
**: K is lowered for CM=350GeV and 500GeV to bring the polarization back to 30%
without adding a photon collimator before the target.
3.3
System Description
The layout of the electron side of the ILC is shown in Figure 3.1, including the relative
position of the major systems of the positron source. Figure 3.2 is a schematic of the
positron source beamlines with dimension indicated, split into two sections. The upper
section shows the beamlines from the end of electron main linac to the end of the
400MeV positron pre-accelerator. The lower section shows the beamlines from the end of
the pre-accelerator to the end of the positron source beamline or the beginning of the
damping ring.
The electron beam from the main linac passes through the undulator and a dogleg before
continuing to the IP for collisions. These beamlines are labeled as EUPM, EUND and
EDOGL in Figure 3.2. For lower energy operations(CM=200GeV, 230GeV and
250GeV), a separate 5Hz 150GeV drive beam is used for positron production. After
passing through the EUND beamline to generate photons, this 150GeV drive beam is
then sent to a beam dump in the beamline EPUNDDL.
The photon beam produced by the electron beam drifts through the section UPT and
strikes into a 1.4cm thick Ti-alloy target to produce an electromagnetic shower of
positrons and electrons. The positrons are then captured withan optical matching device
(OMD) and then matched into a capture system (labeled PTAPA) consisted of normal
conducting (NC) L-band RF cavities and surrounding solenoid. The positron beam is
accelerated to 125 MeV before entering the chicane where the positrons are separated
from the electrons and used photons, into beamlines PCAP, PCAPEDL and PCAUPDL
respectively. Both electrons and photons are dumped. After the chicane, the positron
beam is further accelerated to 400 MeV using a NC L-band RF system with solenoidal
focusing (labeled beamline PPA).
Figure 3.1: Layout of Positron System Relative to the ILC
Figure 3.2: Positron Source Beamlines Cartoon
The 400 MeV positron beam is then transported for approximately 479m in beamline
PTRAN (400MeV) to a booster linac (PBSTR) where the beams are further accelerated
to 5 GeV using SC L-band RF. Before injected to the damping ring, the beam transports
for 903 meters in PTRANH and then passes through a beamline section (PLTR) that
contains energy compression and spin rotations for maximum injection acceptance.
Finally, the beam is injected into the positron damping ring at point TPS2DR.
Showing in Figure 3.3, for the given undulator parameters (K, u), the performance of
the positron source (yield and polarizations) are strongly depend on the the main
electron beam energy. At higher electron beam energy, the undulator B field is reoptimized to restore the polarization to 30% . The final parameters for 350 and 500 GeV
collider are listed in table 3.2 with adjusted undulator parameters.
Figure 3.3. Simulation results of positron source yield and polarization as a function of
drive beam energy for 147m long undulator with K=0.92 and u=1.15cm using flux
concentrator as OMD.
Table 3.2: Parameters for 350GeV CM and 500GeV CM.
Parameter
units
350GeV
500GeV
Electron beam energy (e+ prod.)
GeV
178
253
Bunches per pulse
N
1312
1312
Photon energy (first harmonic)
MeV
16.2
42.8
Photon openning angle (=1/)
r
2.9
2.0
Undulator length
m
147
147
Required undulator field
T
0.698
0.42
undulator period length
cm
1.15
1.15
0.75
0.45
undulator K
Electron energy loss in undulator
GeV
2.6
2.0
Induced energy spread(assume 0% initial)
%
0.122
0.084
Emittance growth
nm
-0.55
-0.31
Average photon power on target
kW
54.7
41.7
Incident photon energy per bunch
J
8.1
6.0
Energy deposition per bunch (e+ prod.)
J
0.59
0.31
Relative energy deposition in target
%
7.20%
5%
Photon rms spot size on target
mm
1.2
0.8
Peak energy density in target
Pol. of Captured Positron beam
J/cm3
295.3
304.3
J/g
65.6
67.5
%
30
30
One additional part of the positron source system is the Keep Alive Source (KAS), which
is not shown in Fig 3.2. The current KAS scheme requires to generate a single bunch low
intensity positron beam (1% of nominal beam intensity) with structure single bunch)
which allow various beam feedbacks to remain active if the main electron beam
producing the undulator based positrons is off. This source uses a S band 500 MeV
electron drive beam from a conventional electron accelerator impinging on the same
production target to produce positrons which then pass through the capture, acceleration
and transport beamlines sections, and then injected into the damping ring .
3.3.1 Photon Production
Production of an adequate number of positrons requires both that the photons hitting on
the target have sufficient intensity and high enough energies to produce ~ 1 – 100 MeV
electron-positron pairs that can escape from the target to be captured. In general this
means photon energies of the order of 10 MeV. The total number of positrons produced
must allow for losses between the target and the IP.
The ILC source of sufficient energy and intensity photons is a helical undulator described
in Section 3.5.1. To generate the necessary photon energy requires a beam of very high
energy electrons. As shown in Figure 3.1, the undulator is installed at the end of the
electron main linac. Above 150 GeV, the production electron beam is used as the drive
beam and passes through the undulator to generate the required photons. At lower
production beam energy, the positron yield is too low and a dedicated 150GeV drive
beam is interlaced with the production electron beam.
In general, a helical undulator generates twice the synchrotron radiation power per period
than the equivalent (same maximum field) planar undulator, reducing the length required
to produce the same number of positrons. Another benefit is that the helical undulator
generates circularly polarized photons which in turn generates longitudinally polarized
positrons. For the baseline undulator system, with 150 GeV drive beam, the produced
photons generate enough captured positrons with ~30% longitudinal polarization. To
achieve higher positron polarization, one needs to increase the undulator length , which
increases the number of photons produced, that will allow photons with the wrong
polarization state to be spatially collimated before hitting the target. Here we consider a
case for an upgrade of 60% positron polarization can be achieved with extension
undulator length of 73.5 meters.
3.3.2 Positron Production & Capture
Figure 3.3 shows the schematic layout for the positron beam production, capture and
transport to the damping rings. . The photon beam generated from the helical undulator is
incident on the rim of a rotating titanium target (see Section 3.5.2) with 0.4 radiation
lengths thickness. The incident photon beam has transverse size of ~ 1 mm rms and the
electron/positron particles emerging from the downstream side of the target are captured
in a 0.07 m-rad transverse dynamic aperture. The target is followed by the tapered
magnetic device called Optical Matching Device (OMD) (see Section 3.5.3) which has a
field varying from < 0.5T at the target and then quickly ramped to over 3T in ~ 2 cm, ,
and then decays from 3-0.5 T over 14 cm. This OMD has wide energy acceptance and is
used to match the beam phase space out of the target into the capture L-band RF cavities
(TAP). The capture RF cavities are placed directly after the OMD to accelerate the
positron beam to 125 MeV. The accelerating RF cavities have an average gradient of 9
MV/m and are located inside 0.5 Tesla solenoids which provide beam focusing. Further
details of the RF are given in Section 3.3.3.
~147GeV e-
PPA(125-400MeV)
Target
PBSTR (Cryo-modules for
boosting energy up to 5GeV)
150GeV e-

helical undulator
Collimator
OMD
TAP
(~125MeV)
 dump
e- dump
Damping ring
Figure 3.3. Schematic layout of Positron Source
The target and equipment immediately downstream of the target will become highly
activated. A remote-handling system is used to replace the target, OMD and 1.3 meter
NC RF cavities. The remote handling system is described indetail in TDR1 Section 5.3.9.
3.3.3 Positron Transport
After the capture section, the positrons are separated from electrons and photons in the
dipole magnet at the entrance of an achromatic chicane which horizontally deflects the
positrons by 1.5 m. The chicane includes collimators to remove positrons with large
incoming angles and large energy errors.
The pre-accelerator immediately downstream of the chicane accelerates positron beam
from 125 MeV to 400 MeV. It consists of normal conducting L-band RF structures
immersed in a constant solenoid field of 0.5 T. The accelerating gradient is designed ~8
MV/m and the total length is 34.6 m.
The transport line is 480m long and transfers the 400MeV positron beam to the positron
energy booster linac.
3.3.4 5-GeV SC Booster Linac
The booster linac accelerates the beam from 400 MeV to 5 GeV using SC L-band RF
modules. There are three sections with periodic FODO lattice. The first low energy
section up to 1083 MeV contains four cryomodules with six 9-cell cavities and six
quadrupoles, instead of the standard ILC cryomodules. The quad’s field strength
(B/x)L is in the range of 0.8-2.4 T. The second section up to 2507 MeV has six
standard ILC-type cryomodules, each containing eight 9-cell cavities and two
quadrupoles. The quad strength is in the range of 0.6-1.4 T. The last section up to 5 GeV
has twelve standard ILC-type RF cryomodules, each with eight 9-cell cavities and one
quadrupole. The quad strength is in the range of 0.8-1.7 T. The total length of the SC
booster beamline is 350m
3.3.5 Linac to Damping Ring Beam Line
The linac to damping ring (LTR) system from the booster linac to the DR injection line
has two main functions: to rotate the polarization into the vertical plane; and to compress
the energy spread to meet the DR longitudinal acceptance.
Fig.3.5 Geometry of LTR beamline. z=0 is at where LTR beamline starts.
The positrons from the target are longitudinally polarized and this polarization is to be
preserved through transport and acceleration. The polarization must be rotated into the
vertical plane to preserve the polarization in the DR. The spin rotation system consists of
bending magnets and solenoids, changing the spin of positrons first from the longitudinal
to horizontal plane and then from horizontal to vertical. To produce n  90 of spin
rotation (n is odd integer) from longitudinal to horizontal plane at 5 GeV, a total bending
angle  bend  n  7.929 is required. To rotate the spin 90 from the horizontal to vertical
plane at 5 GeV energy requires a solenoid magnetic field integral of Bz  Lsole = 26.2
T.m. This is achieved with an 8.3-m-long superconducting solenoid with 3.16 T field.
The energy compression uses a combination of booster linac RF phase, a chicane at the
beginning of the LTR and RF voltage. The chicane has a transverse offset of ~1.5m and a
nominal R56 of -0.75m. The first arc of the LTR has a bending angle of 3  7.929=
23.787 to rotate the spin 90o. After the 1st arc, an RF voltage of 225 MV provided by a 9
cavity RF cryomodule with no quads. This compresses the positron energy to match into
the DR. The rest of the LTR system includes: a section with an additional 9.626
horizontal bending; a vertical dogleg to raise the elevation up by 1.65m; another vertical
dogleg to lower the elevation back down to its final 0.35m; a FODO lattice to transport
the beam and a matching section into the DR injection line. Its geometry is shown in
Figure 3.5.
3.4
Optics Parameters
The optics of the positron source system starting from the capture section to the DR
injection is shown in Figure 3.6. The lattice is optimized to have maximum transmission
and minimum emittance growth.
Fig. 3.6: Optics of positron source
Multi-particle tracking has been performed from the target to the DR injection Using
Elegant [1] to track the large angular divergence and long low-energy tails. Energy
compression is required before injection into the DR to accommodate more positron
beam within the 6-D acceptance in the DR equal to Ax  Ay  0.07 m and E  z 
(37.5MeV)(3.5cm).
3.5
Accelerator Components
3.5.1 Undulator
The undulator uses superconducting technology to achieve high field with a short period.
Two interleaved helical windings of NbTi spaced half a period apart generate the
transverse helical field. The undulator length requires that it be built in modular units.
Each 4m long cryomodule contains two separate undulators with an active undulator
length of ~3.5m.
The present baseline parameters are given in Table 3.2
Table 3.2:
Helical Undulator Parameters
Period (mm)
11.5
K
0.92
Field on Axis (T)
0.86
Beam aperture (mm)
5.85
First Harmonic Energy (MeV)
10.1
Nominal Drive Beam Energy(GeV)
150
The undulator vacuum chamber is made of copper and operates at a temperature of 4.2K.
Copper is selected for its high conductivity which alleviates resistive wall effects. For the
most difficult ILC parameter set (150 m long Gaussian bunch containing 1 x 1010
electrons interacting with a 200m long copper vessel with internal aperture of 5.6mm),
estimates are that the resistive wall effect would only increase the RMS energy spread
from the nominal value of 0.05% to 0.0505%. Another advantage of using copper is that
excellent surface roughness is readily achievable in real copper vessels. A pessimistic
wakefield model has suggested that for a measured surface roughness (RA value) of <100
nm, the electron energy spread would only increase from 0.05% to <0.051%. The
resistive wall wakefield has the potential of causing emittance growth. But numerical
simulations have shown that there is no effect the ILC undulator until the transverse kick
strength is increased to >5000 times the nominal value.
The material between the superconducting windings is soft magnetic iron and this serves
as an outer yoke to increase the field and to provide additional support. Each cryomodule
contains a liquid helium bath and zero liquid boil off is achieved through the use of insitu cryocoolers.
Since the electron vacuum vessel is at cryogenic temperatures, each module effectively
acts as a long cryopump. Roughing pumps are installed in room temperature sections
between cryomodules (approximately every 12 m) but achieving UHV conditions relies
upon cryopumping. The baseline pressure target of 10-8 mbar is set to avoid fast ion
instability problems. Vacuum calculations indicate that the cryopumping is adequate
provided that the number of photons with energy > 10eV striking the vessel surface is
kept low enough. Extensive calculations of the undulator photon output down to these
very low energies have been carried out and these indicate that low power photon
absorbers should be spaced approximately every 12 m to provide an adequate shadowing
of the cold vessel surfaces. These absorbers are in room temperature sections.
The electron beam transport through the complete undulator system is based upon a
simple FODO arrangement with quadrupole spacing of ~12 m (again in the room
temperature sections). There are electron beam position monitors at every quadrupole and
two small horizontal and vertical corrector magnets per cryomodule. Simple electron
beam transport calculations have shown that excellent relative alignment between the
quadrupoles and neighboring BPMs is required. In this simple model, quadrupole to BPM
misalignment of ~5 m leads to an emittance growth of ~2%. It is important to note
however that this is not due to the undulator but to the effect of the quadrupoles and is
therefore a general problem for the ILC beam transport. Sophisticated dispersion free
steering correction algorithms are needed.
3.5.2 Target
The positron production target is a rotating wheel made of titanium alloy (Ti6Al4V). The
photon beam is incident on the rim of the spinning wheel. The diameter of the wheel is 1
m and the thickness is 0.4 radiation lengths (1.4 cm). During operation the outer edge of
the rim moves at 100 m/s. The combination of wheel size and speed offsets radiation
damage, heating and the shock-stress in the wheel from the ~300kW photon beam. A
picture of the conceptual target layout is shown in Figure 3.7. The current design has a
single shaft. The motor is water union on the other end to allow cooling water to be
fed into the wheel. The beam power is too high to allow a vacuum window downstream
of the target. The target wheels sit in a vacuum enclosure at 10-8 Torr (needed for NC RF
operation), which requires vacuum seals for access to the vacuum chamber. The rotating
shaft penetrates the enclosure using one vacuum passthroughs, The optical matching
device (OMD – sees Section 3.5.3), is mounted on the target assemblies, we use rm
temp amd developed at LLNL.. The motor driving the target wheel is sized to overcome
forces due to eddy currents induced in the wheel by the OMD.
The target wheel assembly is designed for an operational life of two years. In the event
that the target fails during a run, the assembly can be replaced by a new assembly in less
than a day using a vertical removable remote handling scheme.
Figure 3.7. Overall Target Layout.
A series of sensors provide information on the target behavior. An infrared camera tracks
temperatures on the wheel, to allow for quick shutdown in the case of a cooling failure.
Flowmeters monitor cooling water flow in and out of the wheel (to watch for leaks), and
thermocouples check ingoing and outgoing flow temperature. There is a torque sensor on
the shaft, and vibration sensors on the wheel to report mechanical behavior. Finally, the
wheel’s rotational speed is monitored.
3.5.3
Optical Matching Device
The OMD generates a solenoidal magnetic field which peaks in strength at 3.2 Tesla
close to the target and falls off to 0.5 Tesla to match the solenoidal field at the entrance of
the capture section. The OMD increases the capture efficiency by a factor of 2. The OMD
is a normal conducting pulsed flux concentrator designed and prototyped by LLNL.
The magnetic field of the OMD interacts with the spinning metal of the target to create
eddy currents. The target design must take into account this drag force which produces
an increased average heat load, requires a stronger drive motor and possibly causes 5Hz
resonance effects.
3.5.4
Normal Conducting RF Accelerator System
Due to the extremely high energy deposition from positrons, electrons, photons and
neutrons behind the positron target, the 1.3 GHz pre-accelerator uses normal conducting
structures up to an energy of 400 MeV. Major challenges are achieving adequate cooling
with the high RF and particle loss heating, and sustaining high accelerator gradients
during millisecond-long pulses in a strong magnetic field. The current design contains
both standing-wave (SW) and traveling-wave (TW) L-band accelerator structures. The
capture region has two 1.27 m SW accelerator sections at 15 MV/m and three 4.3 m TW
accelerator sections at 8.5 MV/m accelerating gradient. The electrons are then accelerated
from 125 MeV to 400 MeV in the pre-accelerator region, which contains eight 4.3 m TW
sections at 8.5 MV/m accelerating gradient. All accelerator sections are surrounded with
0.5 T solenoids. Figure 3.8 shows the schematic layout.
Figure 3.8. Layout of the Capture Region (left) and Pre-Accelerator Region (right).
3.5.4.1
SW Accelerator Structure for Positron Capture
The high gradient (15 MV/m) positron capture sections are simple π mode 11 cell SW
accelerator structures. The advantages are a more effective cooling system, higher shunt
impedance with larger aperture (60 mm), lower RF pulse heating, apparent simplicity and
cost savings. The mode and amplitude stability under various cooling conditions have
been theoretically verified for this type of structure. Figure 3.7 shows a cutoff view of
the SW structure and Table 3.3 gives the important RF parameters.
Table 3.3. Parameters of SW Structure.
.
Figure 3.9. 11–cell SW Structure.
3.5.4.2
Structure Type
Cell Number
Aperture 2a
Q
Shunt impedance r
E0 (8.6 MW input)
Simple π Mode
11
60 mm
29700
34.3 MΩ/m
15.2 MV/m
TW Accelerator Structure for Pre-Accelerator Region
All TW sections are 4.3 m long, 3π/4 mode constant gradient accelerator structures. The
“phase advance per cell” was chosen to optimize the RF efficiency for this large aperture
structure. The advantages are lower pulse heating, easy installation for long solenoids, no
need to use circulators for RF reflection protection, apparent simplicity and cost saving.
Details of the TW sections are given in TDR1 Section 5.3.6.2 and Table 3.4 gives the
important RF parameters.
Table 3.4. Parameters of TW Structure.
Structure Type
Cell Number
Aperture 2a
Attenuation τ
Q
Group velocity Vg/c
Shunt impedance r
Filling time Tf
Power Dissipation
E0 (10 MW input)
3.5.4.3
TW 3π/4 Mode
50
46 mm
0.98
24842 - 21676
0.62% – 0.14%
48.60 – 39.45 MΩ/m
5.3 μs
8.2 kW/m
8.5 MV/m
RF System
Each accelerator section has an individual RF station powered by a 1300 MHz, peak
power 10 MW pulsed klystron. The RF station is composed of modulator, RF windows,
phase shifters, RF loads, directional couplers and low-level RF system. For the SW
structures, RF circulators are needed for reflection protection of the power klystrons.
3.5.5 Magnets
The Positron magnet system has 157 dipoles, 509 quadrupoles and 253 corrector
magnets. The large magnet count is a result of the long beamlines connecting the various
segments of the source. The magnet designs themselves are quite straightforward. In
addition the source uses large aperture DC solenoids, surrounding the L-band capture RF,
for focusing the positrons at low energies. These magnets are normal conducting to
withstand the beam loss in the target hall. In addition there are two SC solenoids for spin
rotation in the PLTR. The three types of solenoids and their parameters are shown in
Table 3.5.
Item
Short Solenoid
Long Solenoid
SC Solenoid
Table 3.5:
Length
ID
m
cm
1.3
36
4.3
31
2.5
6
Solenoid Parameters
Field Range Field, nominal
kG
kG
4-8
5
4-8
5
52.4
52.4
N
#
4
23
2
3.5.6 Diagnostics
The Positron source has the normal complement of beamline instrumentation to measure
orbit, emittance, charge and energy spread. Specialized diagnostics are designed into the
unique positron systems, e.g. target. The major cost is in the BPM system because of the
large channel count coming from the long beamlines. The number of readout channels is
halved by processing only one transverse plane of the BPM x,y pair at each quadrupole.
Performance specifications for the diagnostics are in most cases equal to or less than the
Main Linac or RTML.
3.5.7 Electron & Photon Beam Dumps
There are 9 beam dumps, 16 variable aperture collimators, 1 fixed aperture collimator
and 5 stoppers with burn through monitors planned for the positron source system. Three
of the beam dumps must absorb sufficiently large beam power that they require designs
with water in the path of the beam. The plumbing required to cool and treat the resulting
radioactive water dominates the cost of the dump and collimator technical system in this
area of the ILC.
There is a tune-up dump in front of the undulator at the 150 GeV point of the electron
linac. It is assumed that this dump is only used with a shortened bunch train (100
bunches) at nominal beam parameters and 5 Hz. With these parameters the tune-up dump
must absorb 240kW. This dump, roughly in line with the linac, also serves as the abort
dump for up to a full train of electrons (1.35 MJ) to protect the undulator. The dump
consists of a roughly 40cm diameter by 250cm long stainless vessel filled with 10mm
diameter aluminum balls through which flows approximately 30 gallons per minute of
water; it is backed by a short length of peripherally cooled solid copper. The dump needs
to be shielded from the access passageway by 10cm of steel and 40cm of concrete. A
service cavern is required to house a heat exchanger, pumps and a system to treat the
water for hydrogen and tritium.
A second dump, technically identical (225kW at nominal beam parameters), is required
to tune the 5 GeV positrons before injection into the damping ring.
The most challenging dump in the positron production system is the one that absorbs
non-interacting undulator photons from the positron production target. This dump must
absorb 300kW continuously (2 x 1017 photons/sec of 10 MeV average energy produced
with a 3 microradian angular spread.) The primary absorber in this case must be water,
contained in a vessel with a thin window. Preliminary calculations have shown that, at
the nominal mid-undulator to positron target distance of 500m and the nominal target to
dump separation of 150m, the power density on a 1mm Ti window is 0.5 kW/cm2 and the
resultant temperature rise after the passage of one bunch train of 425 degrees Celsius; in
the core of the beam the rise in the water temperature would be 190 degrees Celsius.
The dump is a compact (10cm diameter by 100cm long) pressurized (12 bar) water vessel
and Ti window, with a radioactive water processing system. It may be possible that
lengthening the target to dump distance to 500m will result in a less technically
challenging and inexpensive system.
The remaining dumps and collimators in the positron system all are based on peripherally
cooled solid metal construction, with the cooling water supplied directly from the
accelerator low conductivity water (LCW) system and do not present a technical or cost
challenge